1. Field of the Invention
The present invention relates to a flow rate detector which is used for measuring the amount of air introduced into, for example, an internal-combustion engine, and other applications, and which is designed to measure the flow velocity or the flow rate of fluid by making effective use of the phenomenon wherein the heat is transferred to the fluid from a heating element or a portion which has been heated by the heating element.
2. Description of Related Art
FIG. 18 is a sectional view showing a conventional bridge-type thermosensitive flow rate sensor disclosed, for example, in Japanese Patent Publication No. 5-7659; FIG. 19 is a top plan view of the flow rate sensor shown in FIG. 18 with a protective film removed. In the drawings, an insulating support film 2 composed of silicon nitride is formed on a flat substrate 1 composed of a silicon semiconductor. Formed on the support film 2 are a heat generating resistor 3, temperature measuring resistors 4 and 5, and a comparison resistor 6 which are composed of permalloys, i.e. thermosensitive resistors. The heat generating resistor 3 is disposed between the temperature measuring resistors 4 and 5; and the comparison resistor 6 is disposed with a predetermined distance from the temperature measuring resistor 4.
Formed on the support film 2 and the resistors 4 to 6 is an insulating protective film 7 composed of silicon nitride. An air gap 8 is provided in a portion of the substrate 1 where the heat generating resistor 3 and the temperature measuring resistors 4 and 5 are located, thus forming a bridge portion 9. The air gap 8 is formed by removing a part of the substrate 1 through an opening 10 by using an etchant which does not attack silicon nitride.
In such a conventional flow rate sensor, the heating current supplied to the heat generating resistor 3 is controlled by a controlling circuit, not shown, so that the temperature of the heat generating resistor 3 is 200 degrees Celsius higher than the temperature of the substrate 1 detected at the comparison resistor 6. Since the air gap 8 is provided under the heat generating resistor 3, the heat generated at the heat generating resistor 3 is hardly transferred to the comparison resistor 6, and the temperature of the comparison resistor 6 becomes nearly equal to air temperature.
The heat generated at the heat generating resistor 3 is transmitted to the temperature measuring resistors 4 and 5 mainly via the support film 2 and the protective film 3. Since the heat generating resistor 3 and the temperature measuring resistors 4, 5 have a symmetrical shape, there is no difference in resistance between the temperature measuring resistors 4 and 5 when there is no air flow. When an air flow is generated, the temperature measuring resistor located on the upstream side is cooled by the air, while the temperature measuring resistor located on the downstream side is not cooled as much as the upstream temperature measuring resistor because the heat is transmitted from the heat generating resistor 3 via the air.
For instance, when an air current is produced in the direction indicated by arrow A in FIG. 19, the temperature of the temperature measuring resistor 4 becomes lower than the temperature of the temperature measuring resistor 5; the difference in resistance between the two temperature measuring resistors increases as the flow velocity increases. Hence, the flow velocity can be measured by detecting the difference in resistance between the temperature measuring resistors 4 and 5. In addition, the flowing direction of a fluid can be also detected by determining which of the temperature measuring resistors 4 and 5 has a lower temperature.
FIG. 20 is a sectional view showing a conventional diaphragm type thermosensitive flow rate sensor; FIG. 21 is a top plan view of the flow rate sensor shown in FIG. 20 with a protective film removed. The portion of the substrate 1 where the heat generating resistor 3 and the temperature measuring resistors 4, 5 are formed has been produced by eliminating a part of the substrate 1 from the rear surface by etching or other means so as to form a diaphragm 11. This diaphragm type flow rate sensor employs the same principle as the bridge type to detect flow rate.
FIG. 22 is a sectional view showing an example of the disposition of a conventional thermosensitive flow rate sensor; the thermosensitive flow rate sensor is used as an intake air volume sensor for an automotive engine. In the drawing, a flow rate detector 14 which has a thermosensitive flow rate sensor 12 and a circuit unit 13 connected to the flow rate sensor 12 is connected to the downstream side of an air cleaner element 15. A throttle valve 16 is provided on the downstream side of the flow rate detector 14.
In the configuration described above, dust 17 in the intake air is trapped by the air cleaner element 15, and the air cleaner element 15 is clogged in the course of traveling, causing the air current to be deflected on the downstream side of the air cleaner element 15. More specifically, the air current mostly flows as indicated by arrow B at the beginning, but it comes to flow as indicated by arrow C or D as dust 17 builds up on the air cleaner element 15. Furthermore, since the throttle valve 16 is located on the downstream side of the flow rate sensor 12, the flow is also deflected on the downstream side of the flow rate sensor 12.
Installing the conventional thermosensitive flow rate sensor thus constructed in a fluid channel causes the air current to separate at the leading edge of the sensor and to generate eddies at random. The generated eddies merge and reattach to the sensor, adding to the disturbance in the flow on the surface of the sensor; therefore, the signal-to-noise ratio drops, making it impossible to achieve satisfactory sensitivity.
A sensor disposed on the inner wall surface of a fluid channel or on the inner wall surface of the casing installed in the fluid channel is disclosed in, for example, Japanese Patent Laid-Open No. 5-142008 or Japanese Patent Laid-Open No. 6-50783. This sensor, however, is failing to provide satisfactory detecting performance when the flow develops deflection as shown in FIG. 22. In general, when the flow is deflected in the fluid channel, the change in the flow velocity on the inner wall of the channel greatly increases in comparison with the change in the flow velocity at the center of the channel. Hence, the flow rate detecting performance is seriously affected when the sensor is mounted on the wall surface of the fluid channel.
Further, a sensor disposed in a flat rectangular casing is disclosed in Japanese Patent Laid-Open No. 6-50783. This sensor, however, is not axially symmetric with respect to the flowing direction of a fluid to be measured; therefore, even when the flow rate of a fluid flowing through the fluid channel remains the same, deflection in the flow causes a change in the flow rate of the fluid running through the casing, depending on the direction of the deflection, thus adversely affecting the flow rate detecting performance.